The NHLBI Twin Study is a longitudinal study of cardiovascular disease (CVD) in 514 pairs of male twins, 254 monozygotic and 260 dizygotic, born in 1917-1927 and 42 to 56 years old when first examined in 1969-1973. Two follow-up examinations, conducted in 1980-81 and in 1986-87, assessed CVD status and collected repeat measurements of physiological, biochemical, and psychosocial risk factors. At the third examination, baseline data on cognitive performance was obtained for 622 individuals including 132 MZ and 134 DZ pairs. The addition of a fourth exam of this cohort will make the NHLBI Twin Study the longest longitudinal study of cardiovascular disease epidemiology in twin subjects, and the first to investigate the contribution of genes and the environment to associations between CVD risk factors, cognitive performance and brain morphology. This project is a fourth examination of the surviving subjects of this cohort to determine relationships between cardiovascular risk factors collected over 24 years of follow-up, changes in cognitive function since the last exam, and brain morphology assessed by magnetic resonance imaging (MRI).